Air bag systems have become more common recently as safety devices for the protection of passengers, and are being used in more locations, not only for a driver but also for passengers, for protection against side collisions (side bags), and for rear seat occupant protection.
With the increase in the locations and number of air bags mounted has come the need to make air bag systems more lightweight and compact, and the various components of the system are now being designed with the intention of achieving a more compact size and lighter weight. The bag bodies of such air bags are also being made lighter in weight by being left uncoated, by reducing the bag volume, and the like.
The inflators that produce the gas for deploying air bags also need to be made more compact and lightweight as set forth in air bags. The base cloth of air bags must have extremely high airtightness in order to reduce the volume of the gas-generating agent accommodated therein and to make more effective use of lower amounts of gas-generating agents. Uncoated base fabrics used for air bag main bodies must thus have a higher weaving density than conventionally used industrial woven fabrics in order to increase air tightness, that is, to reduce air permeability, but this tends to result in stiffer and heavier woven fabric.
The use of finer yarn to reduce the weight of the woven fabric forming the air bag body has been studied in order to achieve a more lightweight bag while circumventing the drawbacks noted above. That is, practical use has been made of air bags constructed of a woven fabric using yarn of 420 denier, which is finer than the conventionally used 840 denier yarn, and uncoated woven fabric for air bags using a raw yarn of 300 to 400 decitex (270 to 360 denier) has been proposed in Japanese Patent Application Publication (Kokai) H3-137245. However, when woven fabrics made using such a fine denier are used as uncoated bags, they must be made into high density woven fabrics in order to reduce the air permeability, not only resulting in extremely stiff material, but also lowering the mechanical properties of the woven fabric itself, such as the tear strength, while failing to produce significant results in terms of a more lightweight and more compact bag. Proposals have also been made to use a finer raw yarn of smaller denier to produce an extremely light woven fabric in order to make an air bag lighter in weight. U.S. Pat. No. 5,482,317 discloses an air bag featuring the use of a woven fabric obtained using nylon 66 with a denier ranging from 45 to 140. These woven fabrics have a basis weight which is half, or even less, that of conventional air bag woven fabrics and the weight-reducing level exceeds the conventional level. However, the mechanical properties of the woven fabric itself are less satisfactory than those of conventional air bag woven fabrics, and the outer periphery of the air bag of the aforementioned invention is made in the form of a rectangle, while the outer periphery is also seamless in an effort to ensure that the bag body has adequate pressure resistance. However, the rectangular bag forms four corners when deployed, and the effects of the corners on passengers and the like must be taken into consideration. In this patent, the failure to ensure the strength of the inflator fitting hole, which is where the greatest collision force occurs during deployment, results in the risk of unsatisfactory mechanical properties in terms of practical performance.
On the other hand, some of the more compact inflators described above increase the temperature of the gas that is generated due to the internal structure, the ignition properties of the gas-generating agent, and the like.
Efforts have thus been made to study coating processes for providing woven fabrics made of finer yarn with heat resistance.
Japanese Patent Application Publication (Kokai) H10-194063, for example, proposes improving the mechanical properties of, and providing heat resistance to, a woven fabric made of relatively fine yarn by using silicone rubber in an amount of 30 to 50 g/m.sup.2 to coat the surface of woven fabric made from 300 to 400 denier yarn and having a cover factor of 1500 to 2500.
Japanese Patent Application Publication (Kokai) H4-352843 proposes an air bag base fabric comprising applying a rubber resin layer of 10 to 90 g/m.sup.2 on at least one side of a woven fabric which has a cover factor of 1000 to 2000 and made of 100 to 500 denier yarn, with a strength of 15 g/d or more, which is at least 50% higher than the strength of conventionally used yarn. In the latter technique, high strength yarn is used to compensate for the insufficient strength of the woven fabric made of fine yarn, and the woven fabric is coated with relatively large amounts of a coating material to ensure airtightness and heat resistance. However, these inventions both relate to air bag base fabrics, with no suggestion of the method for producing an air bag for practical use. Despite the use of a fine yarn and the reduction in the weight of the woven fabric, the use of relatively large amounts of a coating material makes it impossible to obtain a lightweight air bag which can be compactly accommodated as intended in the present invention.